Energy transfer from Ar metastable atom to N2 molecule in DBD-driven microplasma jet

In this paper longitudinal distribution of Ar metastable atom density and its time development have been measured by LAS (laser absorption spectroscopy) in argon microplasma jet and the relationship between its behavior and emission characteristics was studied. Thin Pylex glass tube with inside diameter of 1.25 mm was used for the nozzle and Ar gas was flowed out with flow rate of 1 to 30 L/min. Ar plasma was initiated at the end of the nozzle with DBD using nickel wire electrodes wound on the outside of the nozzle tube with distance between anode and cathode of 10 mm. Applied voltage waveform was rectangular and the typical frequency was 90 kHz and peak voltage was 1.9 kV. Visible plasma jet length was about 1 to 2 cm. The laser absorption signal was observed only at the rise and fall edges of the applied voltage. During the fall edge, a dense spot of metastable atoms propagated towards the downstream and it became under the detection limit at 7 mm from the end of the nozzle at the typical flow rate condition of 2 L/min. Emission spectra measurement showed N₂ 2nd positive band emissions were enhanced from the position of around 7 mm to the downstream, whereas Ar emissions were dominant in the area less than 7 mm. Moreover, ICCD gated images showed the bright part propagated with similar velocity (7 km/s) until the area around 7 mm, then the "bullet-like" bright point became apparent and accelerated (28 km/s). These results support the energy transfer from Ar metastable to N₂ occurs towards the downstream.